1. Field of the Invention
The present invention relates to a transmission/reception element suitable for use as an antenna with which the frequency characteristics can be changed with switch control.
2. Description of the Related Art
In recent years, a transmission/reception circuit is expected to cover a wider range of frequencies and to be ready for diversity and beamforming. Such an expectation thus leads to the increase of the number of antennas for a parallel arrangement. However, since the antenna is a component large in size occupying a large part of the area in the transmission/reception circuit, a larger number of antennas mean a much larger circuit area, and this is not considered desirable. To solve such a problem, an antenna called reconfigurable antenna has been under development. This reconfigurable antenna is provided with a plurality of metal patterns on a dielectric layer each for use as a radiation section (emission/propagation section), for example. These metal patterns are controlled in terms of their electrical coupling by a switch so that the radiation sections can be changed in electrical length.
Such a reconfigurable antenna mainly includes two types, one is the type with which the frequency (radiation frequency) can be controlled through arbitrary switching, and the other is the type with which the antenna directivity can be arbitrarily controlled. The antenna of the type with which the frequency is controlled through switching is described in US2009-0207091, for example, and such an antenna radiates electromagnetic waves at the frequency corresponding to the electrical length of the radiation sections. Generally, antennas radiate electromagnetic waves of frequencies being integral multiples of the base frequency (ω), i.e., ω, 2ω, 3ω, and others, with any one specific electrical length. On the other hand, as is capable of changing the electrical length through switch control, the reconfigurable antenna singly can transmit and receive electromagnetic waves of any frequencies not being integral multiples of each other. This accordingly helps to reduce the size of space needed for placement of antenna.
As an example, “Reconfigurable Antenna Implementation in Multi-radio Platform”, Helen K. Pan, et al. (Intel Corporation, University of Illinois at Urbana-Champaign) describes a reconfigurable antenna being a monopole antenna partially provided with a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) switch. This reconfigurable antenna can be changed in state in response to a control signal coming from the outside, i.e., can be changed between a state 1 (at the frequencies of 0.8 GHz, 0.9 GHz, and 2.4 GHz), and a state 2 (at the frequencies of 1.8 GHz, 1.9 GHz, 2.1 GHz, and 5.0 GHz). Herein, in the state 1, the frequencies of 0.8 GHz and 0.9 GHz are not integral multiples of each other. This is because the reconfigurable antenna is designed to have a wide range of resonance frequencies, and any close frequencies are covered by one resonance frequency.